Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Various radiation compositions and imageable elements containing reactive polymer binders are described in U.S. Pat. No. 6,569,603 (Furukawa) and EP 1,182,033A1 (Fujimaki et al.). The reactive polymer binders include reactive vinyl groups that are pendant to the polymer backbone. Other IR-sensitive compositions are described in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,787,281 (Tao et al.), and U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Patent Application Publication 2003/0118939 (West et al.), and EP 1,079,276A1 (Lifka et al.) and EP 1,449,650A1 (Goto).
Various publications such as U.S. Patent Application Publications 2003/0017411 (Shimada et al.) and 2004/0180289 (Shimada et al.) describe initiator compositions needed to provide free radicals upon imagewise exposure so the imaged (exposed) regions are cured or polymerized so they are insoluble in the alkaline developers. For example, onium salts (including polyvalent onium salts having multiple cation charges and counterions) are used to generate free radicals in the negative-working compositions and elements described in U.S. Pat. No. 6,759,177 (Shimada et al.). U.S. Pat. No. 6,623,910 (Shimada et al.) describes negative-working elements containing onium salts with a counterion having a valency of at least 2.
EP 1,708,023 (Hayashi et al.) describes polyvalent onium salts in its Chemical Formulae 9 and 10 with different cationic groups along with a suitable number of the same counterion.
Problem to be Solved
The various radiation sensitive compositions of the art can readily be used to prepare negative-working imageable elements but they do not always have the desired long press life with small dot gain.